The present invention relates to an image forming apparatus and an image forming system which can be applied to a copying machine and a printer each being equipped with functions to watch the state of winding of a sheet around a drum when transferring a toner image onto the cut sheet, and to carry out sheet feed control, corresponding to the past record of the winding around the drum.
Up to this time, there have been many occasions to use an image forming apparatus such as a copying machine and a printer wherein a large-sized sheet is cut to the prescribed sheet size, and an image is formed on the sheet that has been cut to the prescribed sheet size (hereinafter referred also to a cut sheet). For example, in the case of an image forming apparatus of an electrophotographic type, it is composed of a sheet cassette, an image forming section and a fixing device. In the sheet feed cassette, a plurality of sheets are stored.
The sheet feed cassette naturally stores sheets which are formed by cutting with a blade for cutting use by pressing the blade against a bundled sheets in the fixed direction from the upper surface thereof. It is known that projections called burrs are generated on the sheets which have been cut by a blade for cutting use. It is also known that the burrs are generated in many cases wherein they are generated on the lower side of a cut surface without being generated on the upper surface, when bundled sheets are cut.
A sheet having been fed out of a sheet feed cassette is conveyed to an image forming section. On the image forming section, there are provided an image writing section, a photoconductor drum, a developing section and a transfer section. On the photoconductor drum, there is formed an electrostatic latent image by the image writing section based on, for example, image data, and this electrostatic latent image is developed by the developing section. The electrostatic latent image on the photoconductor drum is developed to be a toner image. The toner image formed on the photoconductor drum is transferred onto a sheet by a transfer section. The toner image transferred onto the sheet is fixed by the fixing device. The sheet having undergone the fixing is ejected.
Each of FIGS. 19A-19C is an illustration showing image forming apparatus 500 relating to a traditional example and an example of transfer for sheet P. The image forming apparatus 500 shown in FIG. 19A is composed of photoconductor drum 41, transfer section 42 and of second sheet feed section 66. In the example of transfer for sheet P, the sheet P is conveyed in the sheet feed direction I shown with an arrow, and a toner image is transferred onto sheet P from photoconductor drum 41, in a space between photoconductor drum 41 and transfer section 42.
Therefore, even when sheet P shown in FIG. 19B is a cut sheet, no phenomenon of winding of sheet P around photoconductor drum 41 is generated, when no burrs exist on a cut surface of cut sheet P (when burrs are not generated). With respect to the cut sheet, there is sometimes an occasion when a large-sized sheet is cut by the use of a laser beam having a prescribed intensity, in addition to a blade for cuffing. In this case, sheet P is conveyed to the fixing device side without winding itself around photoconductor drum 41, as shown in FIG. 19C.
Each of FIGS. 20A-20C is an illustration showing an example of winding of sheet P around photoconductor drum 41. In FIG. 20A, burrs Pb (projections) are generated on a cut surface for cut sheets P, and there is sometimes an occasion when the burrs turn downward. In this case, a phenomenon of winding of sheet P around photoconductor drum 41 as shown in FIG. 20B, tends to be generated. Due to this, existence of burrs Pb causes a clearance between sheet P and transfer belt. Therefore, potential on sheet P on the transfer belt side is lowered, then, voltage between photoconductor drum 41 and sheet P is enhanced to increase adhesion, which makes it difficult to separate sheet P from photoconductor drum 41. Owing to this, a phenomenon that sheet P winds itself around photoconductor drum 41 as shown in FIG. 20C is generated.
Relating to the image forming apparatus that separates a sheet from a photoconductor drum of this kind, Unexamined Japanese Patent Application Publication No. H06-337595 discloses an image forming apparatus equipped with an electrostatic separation control device. This image forming apparatus is equipped with an image forming device, a transfer device and an electrostatic separation control device. The transfer device carries out electrostatic transfer for an image formed on a surface of a recording medium through an image forming device, by charging for a transfer material conveyed properly from the reverse side of the transfer material.
The electrostatic separation control device is equipped with a separation device, a voltage supply device, a distance measuring device and a control device, and the supply device supplies voltage for separation to the separation device provided to adjoin a recording medium and a transfer device. The distance measuring device is arranged on the separation device side for detecting the state of separation of transfer material by the separation device, and it measures an isolation distance between a position of arrangement of the distance measuring device and the leading edge of the transfer material after the separation. Under the assumption of the foregoing, voltage supplied by the voltage supply device is controlled, for the control device to carry out separation of transfer material by the separation device corresponding to the distance for measurement by the distance measuring device. If the electrostatic separation control device is constructed in the aforesaid way, a sheet can be released from a recording medium accurately and surely based on actual state of separation.
However, in the image forming apparatus 500 relating to the traditional example, there is a following problem.
i. When sheet P winds itself around photoconductor drum 41, it is difficult, in many cases, to judge whether the cause of its winding is a phenomenon of curl of sheet P by a conveyance roller or whether the cause of its winding is a bur generated in the direction of cutting of a cut sheet. Therefore, when many cut sheets are used, there is a fear for the occasion where phenomena of winding of a sheet around a photoconductor drum caused by the direction of generation of burrs on cut sheets is increased.
ii. Unexamined Japanese Patent Application Publication No. H06-337595 discloses a technology wherein releasing efficiency for sheets is improved by detecting the state of releasing of a sheet from a recording medium and by controlling voltage for the separation device. However, under the condition of only voltage control, a sheet for which the separation is difficult is still in existence, and a measure for the occasion wherein a cause for winding of a sheet around a recording medium is a bur on a cut surface of a cut sheet becomes difficult. Therefore, there is caused a problem that jam clearing operations to clear a sheet from a recording medium such as a photoconductor drum (hereinafter, referred to as an image carrier) are increased.